The invention relates to semiconductor infrared detectors.
Known photodiodes are realised of a semiconductor material in which an impinging photon causes electrons to pass from their ground state in which they are bound, to an excited state, thereby creating an electron-hole pair liable to generate a current.
The concentration of these electron-hole pairs depends on the amount of light received, which enables realisation of a photodetector. The application of a polarisation voltage allows measuring the concentration of the electron-hole pairs.
Currently, such detectors are manufactured by piling up semiconductors of different band gaps, so that the carriers are trapped in the material of smallest band gap. Usually materials based on semiconductors III-V make the most efficient detectors. These detectors therefore require the handling of specific materials, for example alloys based on gallium arsenide and aluminum non-compatible with the silicon technologies that are now widely spread and mastered. These silicon technologies are the basis of approximately 80% of the components of modern electronics.
The document Applied Physics Letters, Vol. 69, Nr. 1, Jul. 1, 1996, pages 16-18 discloses a MSM-type silicon infrared radiation detector with linear silicon structures on a SOI substrate. The structures are 1.15 xcexcm wide and 6 xcexcm high.
The document Journal of Vacuum Science and Technology A Vol. 10, Nr. 4, July/August 1992, pages 2105-2113 (document D2) describes nano-oxidation of the surface of a silicon layer hydrogenated beforehand, by the tip of a tunnel effect microscope (STM) to form patterns by chemical etching.
The purpose of the invention is therefore the realisation of a semiconductor infrared detector in usual semiconductor technologies, for example the silicon technology.
The goal consists in developing such a photodetector, capable, according to its configuration, to operate over wavelength ranges comprised between 5 and 20 xcexcm.
The purpose of the invention is therefore a semiconductor infrared detector realised with the silicon technology.
Its operation is obtained thanks to the particular geometry of the patterns formed in a layer of silicon. More precisely, its operation is based on the fact that the levels of energy accessible to a particle, more particularly to the electron, are quantified when the space accessible to that particle is confined. The realisation of particular patterns, with nanometric dimensions, makes these levels of quantification variable as a function of the geometric modular in relation to the geometric dimensions and makes them sensitive to the infrared radiation.
According to the invention, this photodetector comprises in that order, a semiconductor substrate, a layer of an electrically insulating material, patterns formed in a layer of semiconductor material. The patterns are formed of at least one island connected to bridges connected themselves with polarisation electrodes. The bridges are lines with width lp approximately constant, whereas the islands are zones with width li larger than that of the lines.
In the preferred embodiments each exhibiting specific advantages:
the detector is such that li is greater than or equal to 1.2 lp, the length of the islands L satisfies the relation 2 lpxe2x89xa6Lxe2x89xa65 lp,
lp ranges between 1 and 10 nm and the detector comprises a number of lines ranging between 1 and 1000, whereas each line comprises approximately 1 to 100 islands,
the patterns formed in the silicon layer are realised by local oxidation of the surface of the silicon layer, then by attacking non-oxidised zones,
local oxidation of silicon is realised using the tip of an A.F.M. or of an S.T.M.
The semiconductor material is advantageously silicon.
The invention also relates to a method for producing such an infrared detector.
According to this method, we start from a structure comprising a silicon substrate carrying a layer of an electrically insulating material on which a silicon layer is superimposed. The patterns are realised by nano-oxidation with the tip of an A.F.M. or of a S.T.M.
In preferred embodiments each exhibiting specific advantages:
Nano-oxidation is realised with a tip of an A.F.M., carried by a periodically oscillating cantilever and local oxidation is obtained by establishing a periodical potential difference between the semiconductor and the tip, whereas the said potential difference consists of synchronised pulses with same period as that of the oscillations of the cantilever. Oxidation is controlled by acting on the phase of the pulses with respect to the phase of the oscillations of the cantilever.
Etching is obtained by a wet process, with KOH, for removing the non-oxidised silicon zones.
The oscillation frequency of the cantilever and of the voltage pulses is approximately of 300 kHz, the mean voltage of the pulses is in the order of 2 volts whereas their peak value is in the order of 10 volts.